"General Finnish Upper Ontology YSO is a trilingual ontology consisting mainly of general concepts. YSO has been founded on the basis of concepts in Finnish cultural sphere. As an indexing tool it is best applicable when indexed material is interdiscliplinary and its themes vary to a great extent.
YSO's hierarchical construction is founded on parent-child relationships. In addition to this, some concepts are supplemented with associative and part-whole relations.
Each concept in YSO is issued to at least one thematic group and some concepts have also been grouped with a non-hierarchical collection label (such as "clothes by materials").
Following the international standards for thesauri, the terms for concepts are usually plural nouns. Terms in singular are usually mass nouns or terms referring to actions or abstract concepts. Some terms carry a different meaning when used in plural and in singular. For example, ballet refers to an art form and ballets to individual works of art.
YSO is based on General Finnish Thesaurus (YSA) and General Finnish Thesaurus in Swedish (Allärs). Concepts in YSO and concepts in YSA and Allärs have been linked to each other with equivalence relationships. YSO has also been linked to Library of Congress Subject Headings (LCSH)."